mongodb C# error on first Push

Question

I've a Parent Entity that has a property of list of Child Entities. Initially on Insert I don't have values for the child entities.

But when I try to Update(by calling push) the document with Child Entities it fails.
This works when I insert a dummy child entity value to the Initial Add .

This is because the embedded document refers to null .

public class ParentDocument : Entity
{

    public string prop1 { get; set; }        
    public List<EmbeddedDocument> EmbeddedDocuments { get; set; }
}
public class EmbeddedDocument
{
    public string prop2{ get; set; }

}

The parent is saved First

_collection.InsertOne(new ParentDocument(){prop1 ="value"});

and later when I Update the document

var builder = Builders<ParentDocument>.Update;
var updateDefintion = builder.Push(x => x.EmbeddedDocuments ,new EmbeddedDocument() { prop2= "value2" });
 _collection.UpdateManyAsync(x=>x.Id==ParentDocumentId, updateDefinition)

error occurs "A write operation resulted in an error mongodb"

But this push works if I have already inserted Embedded Document(s) in the List on first insert.
I think that is because of that the EmbeddedDocuments property is Inserted as null the push doesn't work.

I also tried passing empty List to intial Insert,but not helped.

One Idea would be to check if the count of List of Embedded documents is zero and call

Builder.set(x=>x.EmbeddedDocuments ,new List<EmbeddedDocument>(){ item1 });

But this will cost a query , which I don't want to.

Is there any other solution?

Thanks in Advance

Are you absolutely sure the document you are updating doesn't already have a field that is not an array of the same name for the array property you wish to update? Errors generally occur when this is the case due to a BSON type mismatch. Otherwise a $push operation will just create a new array with the entry (or entries) as supplied. — user3561036
– user3561036, Commented Jun 9, 2015 at 11:09
@Dhanilan What do you mean? You just said in your question that you can set an empty list. — i3arnon
– i3arnon, Commented Jun 9, 2015 at 11:25

Craig Wilson · Accepted Answer · 2015-06-09 13:03:08Z

2

To Hazard a guess, it's because the "array" field in the database is null after the insert. You either need to make the initial value in the database an empty array, or you need to make it not-present. You can either:

use the [BsonIgnoreIfDefault] attribute on your list field to not store nulls,
Initialize your list field to an empty list to store an empty array

This can be reproduced in the shell very easily:

> db.so.insert({x:1, y: null})
> db.so.update({x:1}, {$push: { y: "funny" }})

This will error. However, if you remove y from the insertion or change it to an empty array, the update will succeed.

answered Jun 9, 2015 at 13:03

Craig Wilson

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5 Comments

Dhanilan Over a year ago

thanks. this might be working in shell. But doesn't work in C# api.

Craig Wilson Over a year ago

No, my point is that it doesn't work in the shell either. Did you make one of the changes I suggested in the C# API? Which one did you make? And could you indicate what your documents look like just after insert?

Craig Wilson Over a year ago

I just did both and both suggestions work... perhaps you could post what you are trying. Here is the gist of the second option (using scriptcs): gist.github.com/craiggwilson/f7145d6b00beaf9d0bdb

Craig Wilson Over a year ago

I think I know what might be your problem... You already have this data in the database. To do this update, you are going to need to fix the documents that have null values for this field.

Dhanilan Over a year ago

hey craig...that works :). My case was though i passed empty List in constructor it was being over ridden when copying from my viewmodel. your gist helped me. Thanks a lot. I just added BsonIgnoreIfNull to the property.started to work.

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